A. 4cm.
B. 16cm.
C. 25cm
D. 5cm
\[d=[2k+1].\frac{v}{2f}\Rightarrow v=\frac{2df}{2k+1}\]
\[\Rightarrow v=\frac{2.10.20}{2k+1}=\frac{400}{2k+1}[\frac{cm}{s}]\]
Mà \[80\leq v=\frac{400}{2k+1}\leq 100\Rightarrow \left\{\begin{matrix} v=80\frac{cm}{s}\\ k=2 \end{matrix}\right.\]
\[\Rightarrow \lambda =\frac{v}{f}=\frac{80}{20}=4[cm]\]
Chọn A
\[\Rightarrow v=\frac{2.10.20}{2k+1}=\frac{400}{2k+1}[\frac{cm}{s}]\]
Mà \[80\leq v=\frac{400}{2k+1}\leq 100\Rightarrow \left\{\begin{matrix} v=80\frac{cm}{s}\\ k=2 \end{matrix}\right.\]
\[\Rightarrow \lambda =\frac{v}{f}=\frac{80}{20}=4[cm]\]
Chọn A
Nguồn: Học Lớp